1. FIELD OF THE INVENTION
The present invention relates to a zoom lens barrel, and more precisely it relates to a position control mechanism and an advancing mechanism of a zoom lens.
2. DESCRIPTION OF RELATED ART
A zoom lens has at least two groups of lenses which are independently moved in the optical axis direction. At least one of the groups of lenses is moved non-linearly in the optical axis direction to change the focal length of the group(s) of lenses while keeping a constant focus point. On one hand, a multiple thread is used as means for providing linear movement, and on the other hand, means for providing non-linear movement is usually constituted by a cam mechanism which comprises a cam groove provided on the cylindrical wall of the barrel and a guide pin provided on a lens frame which is fitted in the cam groove so that the lens frame is guided in the optical axis direction.
The wall surface of the cam groove extends in the same direction as that of the guide pin at any place of the cam groove. Generally speaking, the guide pin extends in a radial direction from the center of the barrel, and accordingly the wall surface of the cam groove must be formed to extend toward the center of the barrel at any place thereof. A cam groove having such a profile must be machined.
However, the machining of the cam groove is very expensive. Multiple grooving of the cams having different complex cam profiles which cause the lenses to move non-linearly is even more expensive. One solution to a reduction of the manufacturing cost is to make the cam groove by plastic molding.
However, when the cam groove is formed on the cylindrical wall of the barrel, a molding die for molding the cam groove must be released or removed in a direction perpendicular to the axis of the barrel. As a result, the direction of the cam groove is parallel with the direction of removal of the molding die along the whole length of the cam groove, and accordingly, the cam groove can not be oriented toward the center of the barrel along the whole length, thus resulting in failure to an engagement of the guide pin projecting from the center of the barrel with the associated cam groove.
Furthermore, in a conventional zoom lens barrel, in case of a large movement of the lens groups, particularly the first lens group thereof, it is necessary to increase the axial length of a stationary barrel in order to increase the displacement of a linear movement ring which supports the first lens group, resulting in a large lens barrel in the axial direction.